The present invention relates to an image forming apparatus employing an electro-photographic method, and in particular, to an image forming apparatus including a toner recycle mechanism in its developing means.
Recently, in image forming apparatuses employing the electro-photographic method, a toner recycling method is generally used, wherein for the purpose of using toner more effectively, resided toner on an image carrier after the image formation is removed by a cleaning device, and the resided toner (hereinafter referred to also as “recovered toner”) is conveyed to the developing means through a feed screw pipe, then the resided toner is used again in the developing process.
In order to repeatedly use recycled toner, concerning the image forming apparatus employing the toner recycling method, incorporated is the toner recycling mechanism by which the recovered toner is returned to the developing device as the developing means, and thereby the condition maintaining predetermined toner density is preserved.
However, when image formation of an area where the number of characters is very low, is greatly performed, very little toner is consumed in the developing device, resulting in an increase of recovered toner at a low transferring ratio, and thereby, very little new toner is supplied from the toner replenishing container.
Under the above described condition, since toner in the developing device, and particularly the recovered toner are agitated in the developing device for a long time, electrostatic charging capability of the toner is largely deteriorated due to stress caused by the agitation.
The deterioration of electrostatic charging capability of toner is outlined below. That is, in order to increase the electrostatic charging capability of toner, while toner is produced, silica is added to cover the surface of toner which increases the flow-ability of the toner. However, silica is stressed during agitation in the developing device, then silica is separated from toner or buried in toner resin, and the deterioration of electrostatic charging capability of toner results.
When toner exhibits the deteriorated electrostatic charging capability, the image transferring efficiency is reduced, resulting in a poor formation of the image, such as roughened images.
In order to prevent poor formation of images, in the image forming apparatus employing the toner recycling method, a method is disclosed wherein the recovered toner is discarded from the toner recycling path which circulates toner to the developing device, based on the developing condition.
For example, in the image forming apparatus, disclosed are:
a technology wherein when a main switch is turned on, or when a copy operation is started after a predetermined period of the non-operation, the apparatus mode is changed to one which discards the toner recovered during a definite period of time;
a technology wherein the density of resided toner on the surface of image carrier is detected as a predetermined value, and the apparatus mode is changed to discard the recovered toner (see Patent Document 1); and
a technology wherein when a temperature value detected by a thermo sensor is greater than a predetermined temperature value, the apparatus mode is changed to discard the toner recovered based on the fact that a double sided copy has been conducted, as well as the operation time of the double sided copy (see Patent Document 2).                Patent Document 1: Japanese Patent No. 3204076        Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-150016        
However, the object of the above technologies is to prevent poor formation of images due to deterioration of the toner characteristic caused by excessive heat. The method for changing the apparatus mode based on the resided toner in the transferring process is to measure the resided toner on the image carrier by the image density sensor. However it is very difficult to exactly measure the residual toner on the image carrier in the transferring process, because the measurement is greatly dependent upon the relationship between the outputted image and the sensor position.